1. Field of the Invention
This invention relates to the ellipsometric analysis of samples enclosed within a small cell, and more particularly to a window structure for the cell that allows accurate ellipsometric measurements to be taken despite the presence of reflections from the window surfaces.
2. Description of the Related Art
Ellipsometry is an established nondestructive optical technique for characterizing the properties of surfaces and surface films, such as film thickness, refractive index, surface oxidation, surface reaction kinetics, catalysis, electrochemistry, corrosion, passivation and anodization. It is described for example in Spanier, "Ellipsometry--A Century Old New Technique", Industrial Research, September 1975, pages 73-76. The technique involves directing an elliptically polarized light beam onto the surface of a sample to be analyzed, and detecting the beam reflected from the surface to determine changes in its polarization state; such changes correspond to the properties of the sample surface. Measurements are obtained for tan .PSI., the change in the amplitude ratio of the parallel component to the perpendicular component of the light wave upon reflection, and .DELTA., the change in the phase difference between the parallel component and the perpendicular component of the light wave upon reflection. The quantities .PSI. and .DELTA. are functions of the surface's optical constants, the wavelength of the light used, the angle of incidence, the optical constants of the ambient medium and, for a film covered surface, the thickness and optical constants of the film. .PSI. and .DELTA. are both measured in degrees, and each different combination of these two quantities corresponds to a unique set of surface conditions.
A conventional ellipsometric measurement system is illustrated in simplified form in FIG. 1. A laser 2 generates a beam 4 that is linearly polarized by a polarizer 6, with the linear polarization indicated by polarization vector 8. The beam is typically oriented at about 70.degree. to vertical, and passes through a compensator 10 that converts it to an elliptical polarization, as indicated by the polarization ellipse 12.
The elliptically polarized beam is reflected off the surface of a sample 14, which is assumed in this illustration to be horizontal, and which is shown as having a surface film 16, and is transmitted through an aperture 18 to an analyzer 20. The analyzer is a high quality crystal polarizer that determines the plane of polarization of the reflected linearly polarized light. The process of reflection changes the beam's polarization in accordance with the film thickness and the optical characteristics of the film and sample. To make a measurement, the polarizer 6 is adjusted in such a way that the combined effect of the polarizer, compensator, sample, and film causes the beam entering the analyzer to be linearly polarized, as indicated by linear polarization vector 22. A filter 24 eliminates unwanted background light from the beam transmitted through the analyzer 20 so that measurements can be made in normal room conditions, with the filtered beam sensed by a photodetector 26. The photodetector 26 transmits an electrical signal corresponding to the beam intensity to an extinction meter 28.
There are certain settings of the polarizer that cause the beam reflected from the specimen to be completely linearly polarized. At such settings the analyzer 20 can be rotated to a position at which almost no light reaches the photodetector 26, and the extinction meter 28 moves to its lowest reading. Measurements are taken at two such settings, from which the film thickness, refractive index and other characteristics can be determined with the use of graphs, tables or calculators.
In certain applications it is desirable to obtain ellipsometry measurements within a controlled environment. For such cases several techniques have been used to allow a sample to be analyzed without exposing it to the environment in which the analysis equipment is located; three such arrangements are illustrated in FIGS. 2, 3 and 4. In FIG. 2 a specimen 30 to be analyzed is placed within a sealed chamber 32. Beam entrance and exit ports 34 and 36 into the chamber have respective transparent windows 38 and 40 that allow an analysis beam to be transmitted into and reflected out of the chamber, without impairing the sealed environment within the chamber. Apparatus comparable to that shown in FIG. 1 is used to produce an elliptically polarized entrance beam 42 that is directed through the entrance window 38 onto the specimen 30, and to analyze the exit beam 44 reflected off the specimen. The beams 42 and 44 are generally transmitted at right angles to the surfaces of their respective windows 38 and 40.
In FIG. 3 a sealed chamber 46 is shown with only a single entrance/exit port 48, and a transparent window 50 sealing the port. An elliptically polarized entry beam 52 is transmitted through the window 50 and reflected off the specimen 30 within the chamber. The reflected beam is redirected back onto the specimen by a mirror 54, but at an altered angle so that it reflects off a different portion of the specimen to exit from the chamber as exit beam 56. The entry and exit beams 52 and 56 are offset from each other both spatially and angularly, and the analysis of the exit beam is modified to account for the double reflection off the specimen.
In FIG. 4 another chamber 58 is shown with open entry and exit ports 60 and 62, respectively. This arrangement is similar to that of FIG. 2, except the transparent windows 38 and 40 of FIG. 2 are omitted and a gas inlet port 64 is provided in the chamber 58. An inert gas such as nitrogen (indicated by arrows 66) is admitted into the chamber under pressure through gas port 64 and flows out of both beam ports 60 and 62. This outward flow of inert gas allows ellipsometric analysis to be performed, while effectively sealing the interior of the chamber and the specimen 30 from the outside environment.
A more recent application for ellipsometric measurements concerns monitoring the surface condition of a sample contained within a small volume cell. The purpose of this application is to minimize the volume of wet chemical reagents used in the fabrication of microelectronic circuits. It is illustrated in FIG. 5, and involves the provision of a small volume cell 68 that houses a semiconductor wafer 70, or a portion of a wafer, for chemical processing prior to the fabrication of microelectronic circuitry on the wafer. Such preparation normally involves cleaning, etching, and other wet chemical processing.
The specially designed cell 68 of FIG. 5 substantially reduces the volume of wet chemicals required for the processing, and is the subject of copending patent application Ser. No. 899,792 filed Jun. 19, 1992 by Gerald A. Garwood, Jr., a co-inventor of the present invention now U.S. Pat. No. 5,265,960. U.S. Pat. No. 5,265,960 is assigned to the Santa Barbara Research Center, the assignee of the present application. The wafer 70 is shown supported directly by the cell base 72, although it may alternately be supported off the base by means of standoffs if processing of both faces of the wafer is desired. The cell 68 is covered by a flat transparent lid 74, which is preferably glass but might alternately be formed from a transparent plastic that does not react with the chemicals used in the substrate processing. The lid 74 is fastened to the base 72 by means of bolts 76 around its edge, with an O-ring 78 sandwiched between the lid and base to seal the interior of the cell. The wet chemicals used to treat the wafer are cycled through the cell by means of an inlet port 80 and an outlet port 82 that extend through the base from the exterior of the cell to a location inward of O-ring 78. The clearance between lid 74 and wafer 70 is kept quite small, as is the peripheral spacing between wafer 70 and O-ring 78, thereby greatly reducing the volume of wet chemicals that would otherwise be required to prepare the wafer.
The chemical processing cell 68 is at least theoretically adapted to ellipsometry measurements to monitor the wafer surface at various stages of the processing. In principal, this can be accomplished by directing an elliptically polarized beam 84 at a angle through the lid 74 and onto the upper surface of the wafer 70, with the beam reflecting off the wafer and proceeding back through the lid for analysis; the actual ellipsometry equipment is not shown in FIG. 5, but it would be comparable to that illustrated in FIG. 1. Although the beam 84 is refracted during both passes through the lid 74, these angular deviations cancel each other and the exit beam emerges from the lid at the same angle to vertical as the entry beam (assuming the surfaces of the lid and wafer are parallel). This is important when using the cell 68 in a standard ellipsometry setup, in which the beam entry and exit angles are generally fixed.
A problem in making ellipsometry measurements with the described small volume cell is illustrated in FIG. 6, which shows the elliptically polarized beam 84 incident on the upper surface of the flat transparent lid 74. This incoming beam gives rise to a set of parallel rays that emerge from the cell, due to partial reflections at the upper (outer) and lower (inner) lid surfaces. The primary reflections are an initial reflection of the incoming beam from the lid's upper surface (ray 86), a reflection of the incoming beam from the lid's lower surface, after undergoing refraction at the upper surface/air interface (ray 88), and a double reflection of the outgoing beam from the lid's upper and lower surfaces, after refraction at the lower surface/air interface (ray 90). The outgoing beam itself after reflection from the wafer 70 is indicated by ray 92. Additional parallel rays that emerge from the lid due to multiple reflections are also present, but are of much lower intensity. Because of the small vertical spacing between the lid 74 and wafer 70 to minimize the volume of chemical reagents used, the principal rays 86, 88 and 90 and outgoing beam 92 are grouped close together and two or more of them can enter the analyzer aperture. However, it is only beam 92 that carries the desired information which represents the characteristics of the wafer 70. The other rays carry conflicting and unwanted information that interfere with a proper measurement of the sample within the cell.
The upper and lower lid surfaces could be coated with an antireflection coating to reduce or even eliminate the undesired reflections. However, an antireflection coating would produce further changes in the polarization state of the light passing through it, and the coating on the bottom surface of the lid would be exposed to all the chemical reagents that flow through the cell to treat the wafer. The addition of antireflection coatings is thus not a viable solution. An alternate approach would be to increase the spacing between the reflected rays and the principal ray 92 enough to keep the reflected rays out of the analyzer aperture by substantially increasing the distance between the wafer and the cell lid. This, however, would greatly increase the cell volume and thereby defeat the purpose of having a small cell.